Free motion sewing, also called free motion quilting, is performed by an operator with a sewing machine set up with the material transportation device (e.g., feed dogs) in the lower arm of the sewing machine, disabled or non-existent. This allows sewing in any direction the material may move. Stitch length is then controlled by the operator depressing a standard foot control (such as a treadle starter) and moving the material under the needle at a rate which will create the desired stitch length. By relying only on operator control, the stitch length can be inconsistent and uneven. To regulate stitch length, imaging devices have been used on top of the material to determine the position of two adjacent stitching sites of the sewing needle on the article being sewn.
One known method of stitching control uses an optical device for detecting the motion of the top of the article being sewn (i.e., fabric or other material) relative to the machine and so regulates the stitch length by controlling the needle movement. This method places the detector as close as possible to the needle and may be held on a mechanism that moves up and down relative to the article being sewn. This method experiences several different problems. First, optical device technology as described requires that the object being detected be held at a distance from the sensor that does not vary except within a very specific distance range from the optical sensor for accurate detection. This distance cannot be guaranteed when quilting is performed on a stack of material, often consisting of two or more pieces of material with a thick or fluffy batting between them. The stitches already placed tend to compress the batting around the area where they are placed, making the material stack thinner in those areas. The resistance to compression of the batting will make other areas thicker. The distance may be further increased as the mechanism moves the sensor up and down relative to the material. This difference in thickness can easily exceed the optical focal range of image sensors regardless of optics, causing them to provide unreliable movement data for controlling the movement of the needle. If a means is provided to compress the material stack around the sensor detection area to limit the distance variations, this compression device creates resistance to free movement of the material as it is fed into the needle, also creating the potential for unreliable movement data.
Also, during the portion of the stitching cycle when the needle is inserted into the material for the purpose of making the stitch, the material around the needle will not move laterally at the same rate as the rest of the article. A sensor placed at or near the needle as described will not detect material motion during this time, and therefore will not properly control the needle movement. Since no material motion is detected, the needle can hang in the material and the sewing operation will cease. This creates uneven stitches, defeating the purpose of the device.
Further, color variations in the fabric may fool an optical sensor and create incorrect motion data, and therefore, can cause uneven stitches.
Additionally, optical detection of a material stack being sewn by a sensor located next to the needle and viewing the top of the material as described produces inconsistent movement data and therefore inconsistent stitch lengths.
It would be an advance in the art to provide a device that more reliably detects movement of the material and more reliably controls the stitching during free motion sewing.